Objects such as wafers are manufactured by highly complicated manufacturing processes. These manufacturing processes should be monitored in order to ensure the quality of the wafers.
The monitoring process may include a first phase of optical, ultraviolet, deep ultraviolet or extreme ultraviolet inspection to detect potential defects and a second phase of defect review.
Defect review is usually executed by a scanning electron microscope (SEM) that exhibits a very high resolution (nanometric order). A SEM usually acquires multiple images of the same area (multiple consecutive scans of the area) and then averages these multiple images to provide an output image of the area. The output image can be displayed to an operator. The operator is expected to evaluate the area in view of the output image.
A high aspect ratio holes such as a deep trench or a deep void, may have an aspect ratio exceeds 20. The high aspect ratio and the small width of the high aspect ratio hole cause the bottom of the high aspect ratio hole to exhibit a very low electron yield. The electron yield of the bottom of the high aspect ratio hole is the ratio between (a) a number of electrons that originate from the bottom of the high aspect ratio hole and are detected by the SEM as a result of an irradiation of the high aspect ratio hole and (b) the number of electrons that irradiate the high aspect ratio hole.
Other non-limiting examples of low electron yield structures include vertical NAND memory structures and trenches.
A most common imaging mode of a SEM includes collecting secondary electrons. These secondary electrons are accelerated and are detected by detector that may include a scintillator (for converting a secondary electron to a photon) and a photomultiplier (for amplifying the photon and outputting an electrical signal). The acceleration process, the conversion of a secondary electron to a photon and the amplification of the photon are noisy and inconsistent analog processes that result in a noisy and fluctuating analog signal of low signal to noise ratio.
There is a growing need to improve the review of low electron yield regions such as but not limited high aspect ratio holes.